Flexible electrostatographic imaging members are well known in the electrostatographic marking art. Typical flexible electrostatographic imaging members include, for example, (1) electrophotographic imaging members (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems and (2) electroreceptors, such as ionographic imaging members for electrographic imaging systems. The flexible electrostatographic imaging members can be in the form of seamless or seamed belts. Typical electrophotographic imaging member belts comprise a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating applied to the opposite side of the supporting substrate layer to induce flatness. Electrographic imaging member belts, however, typically have a more simple material structure, including a dielectric imaging layer on one side of a supporting substrate and an anticurl back coating on the opposite side of the substrate. While the scope of embodiments covers an improved preparation process for flexible electrostatographic imaging members producing a crack resistance enhanced outer top imaging layer, the following discussion will focus only on processing of flexible electrophotographic imaging members for simplicity.
Electrophotographic flexible imaging members typically comprise a photoconductive layer, which can include a single layer or composite layers. Since typical electrophotographic imaging members can exhibit undesirable upward imaging member curling, the anticurl back coating brings each imaging member to at least a desired flatness.
One type of composite photoconductive layer used in electrophotography, illustrated in U.S. Pat. No. 4,265,990, for example, the disclosure of which is hereby incorporated by reference, has at least two electrically operative layers. One layer comprises a photoconductive layer that can photogenerate holes and inject the holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer sandwiched between the contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. The supporting electrode can still function as an anode when the charge transport layer is sandwiched between the supporting electrode and the photoconductive layer. The charge transport layer in this case must be able to support the injection of photogenerated electrons from the photoconductive layer and to transport the electrons through the charge transport layer. Photosensitive members having at least two electrically operative layers can provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member, such as paper.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have created stringent requirements including narrow operating limits on photoreceptors. For flexible electrophotographic imaging members having a belt configuration, the numerous layers found in modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor belt that has been employed as a belt in negatively charging electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer, and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor belt can also comprise additional layers, such as an anticurl back coating to balance curl and provide the desired belt flatness.
In a machine service environment, a flexible multilayered photoreceptor belt, mounted on a belt supporting module that includes a number of support rollers, is generally exposed to repetitive electrophotographic image cycling, which subjects the outer-most charge transport layer to mechanical fatigue as the imaging member belt bends and flexes over the belt drive roller and all other belt module support rollers. The outer-most layer also experiences bending strain as the backside of the belt makes sliding and/or bending contact above each backer bar's curving surface. This repetitive action of belt cycling leads to a gradual deterioration in the physical/mechanical integrity of the exposed outer charge transport layer, leading to premature onset of fatigue charge transport layer cracking. The cracks developed in the charge transport layer as a result of dynamic belt fatiguing are found to manifest themselves into copy print defects, which thereby adversely affect the image quality on the receiving paper. In essence, the appearance of charge transport cracking cuts short the imaging member belt's intended functional life.
When a production web stock consisting of several thousand feet of coated multilayered photoreceptor is obtained after finishing the charge transport layer coating/drying process, it is seen to spontaneously curl upwardly. Hence, the anticurl back coating is applied to the backside of the substrate support, opposite to the side having the charge transport layer, to counteract the curl and render the photoreceptor web stock flatness. The exhibition of upward photoreceptor curling after completion of charge transport layer coating results from thermal contraction mismatch between the applied charge transport layer and the substrate support under the conditions of elevated temperature heating/drying the wet coating and eventual cooling down to room ambient temperature. Since the charge transport layer in a typical photoreceptor device has a coefficient of thermal contraction approximately 2 to 5 times larger than that of the substrate support, upon cooling down to room ambient, greater dimensional contraction occurs in the charge transport layer than in the substrate support. This yields the upward photoreceptor curling of the web stock.
Although, in a typical photoreceptor belt, it is necessary to apply an anticurl back coating to complete a typical photoreceptor web stock material package having the desired flatness, nonetheless the application of the anticurl back coating onto the backside of the substrate support (for counter-acting the upward curling and render photoreceptor web stock flatness) has caused the charge transport layer to instantaneously build-in an internal tension strain of from about 0.15% to about 0.35% in its coating material matrix. After converting the production web stock into seamed photoreceptor belts, the internal built-in strain in the charge transport layer is then cumulatively added to each photoreceptor bending induced strain as the belt flexes over a variety of belt module support rollers during photoreceptor belt dynamic cyclic function in a machine. The consequence of this cumulative strain effect has been found to cause the acceleration and early onset of photoreceptor belt fatigue charge transport layer cracking problem. Moreover, the cumulative charge transport layer strain has also been identified as the origin of the formation of bands of charge transport layer cracking when the photoreceptor belt is parked over the belt support module during periods of machine idling or overnight and weekend shut-off time, as the belt is under constant airborne chemical vapor and contaminants exposure. The bands of charge transport layer cracking are formed at the sites corresponding to photoreceptor belt bending over each of the belt supporting rollers. The crack intensity is also seen to be most pronounced for the band at the belt segment bent and parked directly over the smallest roller, since according to the fundamentals of material mechanics, the smaller the roller diameter the belt segment is bent over, the greater is the bending strain induced in the charge transport layer surface.
Thus, there is a need for a method of fabrication of improved flexible seamed photoreceptor belts, having a charge transport layer with little or no built-in internal tension and reduced bending strain as the belts flex during machine function or during static bent belt parking over the belt module support rollers under the periods of machine idling and shut-off. Such belts will enjoy extended mechanical functioning life and effect the suppression of premature onset of charge transport layer cracking problem as well.
U.S. application Ser. No. 09/973,351, filed Oct. 8, 2001, entitled STRESS RELEASE METHOD (D/A1414), and U.S. Pat. Nos. 5,606,396, 5,089,369, 5,167,987, and 4,983,481, the disclosures of which are hereby incorporated by reference, represent prior efforts toward alleviating the problems discussed above. These efforts yielded were successful to a point. However, resolution of one problem had often been found to create new ones. For example, charge transport layer cracking life extension through selection of a supporting substrate.
Thus, there is a continued need to improve the methodology for cost effectual production of flexible imaging members, particularly through innovative processing treatment approaches that effect charge transport layer internal tension strain reduction or elimination, as well as reduction the bending/flexing strain over belt module support rollers, in multilayered electrophotographic imaging member web stocks to yield mechanically robust imaging member belts.
Embodiments thus provide improved methodology for fabricating multiple layered electrophotographic imaging member web stocks that overcome the above noted deficiencies. For example, embodiments provide an improved process for carrying out flexible electrophotographic imaging member web stocks treatment. Additionally, embodiments provide an improved and refined methodology for processing flexible multilayered electrophotographic imaging member web stocks to effect reduction of charge transport layer internal strain. Advantageously, embodiments provide an improved and refined methodology for processing flexible multilayered electrophotographic imaging member web stocks to effect reduction of charge transport layer bending strain that is induced when imaging member belt flexes or parking over belt support rollers to thereby extend the mechanical service life of the imaging member.
An improved flexible multilayered electrophotographic imaging member web stock results from embodiments. Such web stock has a charge transport layer with reduction of both internal and bending strains for effectual suppression of early onset of imaging member belt charge transport layer cracking problem caused by dynamic belt fatigue during machine belt function or induced as a result of chemical contaminants exposure at the period belt parking when machine idling or shut-off.
Embodiments thus provide an improved treatment process for carrying out multilayered flexible electrophotographic imaging member web stock charge transport layer internal stress reduction that effects the elimination of the need of an anticurl back coating from the imaging member. Additionally, embodiments provide an improved flexible multilayered electrophotographic imaging member web stock having a strain/stress reduction charge transport layer through implementation of invention cost effective web stock stress-releasing treatment production process. A typical web stock comprises a flexible substrate support layer coated over with an electrically conductive ground plane, a hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an anticurl back coating.
A stress-release process has improved and refined features for effectual heat treatment of electrophotographic imaging member web stock to substantially eliminate the internal tension strain from the charge transport layer material matrix, as well as to reduce bending strain prior to fabrication into flexible imaging member belts. To achieve this, embodiments direct the imaging member web stock is directed, with the transport layer facing outwardly, toward the surface of a circular metallic tube making entering contact at 12 o'clock with the tube, heating the transport layer surface to a temperature above its glass transition temperature (Tg), then cooling the web stock to a temperature below the Tg just before the web stock leaves the tube to complete imaging member web stock stress release processing treatment. Embodiments are equally applicable for fabricating electrographic imaging members as well (e.g., ionographic members).
The stress release treated flexible electrophotographic imaging member web stock is then formed into seamed flexible belts that generally comprise a flexible supporting substrate having an electrically conductive surface layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, a ground strip layer, and may or may not need an anticurl back coating. The flexible substrate support layer should be transparent, and can have a thickness of between about 25 μm and about 200 μm. A thickness in the range of from about 50 μm to about 125 micrometer gives better light transmission and substrate support layer flexibility. The conductive surface layer coated over the flexible substrate support can comprise any suitable electrically conductive material such as, for example, aluminum, titanium, nickel, chromium, copper, brass, stainless steel, silver, carbon black, graphite, and the like. The electrically conductive surface layer coated above the flexible substrate support layer may vary in thickness over a substantially wide ranges depending on the desired usage of the electrophotographic imaging member. However, from flexibility and partial light energy transmission considerations, the thickness of the conductive surface layer may be in a range from about 20 Å to about 750 Å. It is, nonetheless, desirable that the conductive surface layer coated over the flexible substrate support layer be between about 50 Å and 120 Å in thickness to provide sufficient light energy transmission of at least 20% transmittance to allow effective imaging member belt back erase.
In the drawings and the following description, like numeric designations refer to components of like function.